A known type of riding lawn mower has the four wheels driven by respective hydraulic motors, with the rear wheels steering the lawn mower. When the lawn mower is traveling in a straight line, all four wheels move along the ground at the same speed (although the rotational speed of the rear wheels may be greater because the rear wheels are smaller). When the lawn mower is turning, however, the wheels do not move along the ground at the same speed. For example, when the lawn mower is turning to the left, the right front wheel moves faster than the left front wheel, due to the greater distance from the center of the turn, and the right rear wheel moves faster than the left rear wheel, for the same reason. Broadly stated, the outside wheel in a turn must move faster than the corresponding inside wheel.
Furthermore, because the rear wheels are turning, the distance from the left rear wheel to the center of the turn is greater than the distance from the left front wheel to the center of the turn, and the distance from the right rear wheel to the center of the turn is greater than the distance from the right front wheel to the center of the turn. Consequently, the steering left rear wheel moves faster than the non-steering left front wheel, and the steering right rear wheel moves faster than the non-steering right front wheel.
The opposite is true for a front-wheel-steering vehicle. That is, in a turn, the left front wheel moves faster than the left rear wheel, and the right front wheel moves faster than the right rear wheel. Broadly stated, the left steering wheel moves faster than the left non-steering wheel, and the right steering wheel moves faster than the right non-steering wheel.
To accommodate an increased speed of a wheel in a turn, whether the wheel is an outside wheel or a steering wheel, the hydraulic motor associated with the faster moving wheel must operate at a faster rate. To do so, this hydraulic motor requires more hydraulic fluid than the hydraulic motor associated with the slower moving wheel requires.
In a conventional lawn mower, all four hydraulic motors are connected in parallel, which will allow for the difference in speed. However, if any one wheel loses traction, all fluid flows to that wheel, causing the lawn mower to stop moving.
If the hydraulic motors are arranged in a series circuit, the hydraulic motor associated with the wheel which must move faster in a turn does not itself operate faster, the associated wheel, in an effort to rotate faster, will exert pressure on the motor. The rotation of the associated wheel will be impeded by the motor, and, because the wheel is not rotating faster, the wheel will be dragged by the other wheel, which can move slower because this other wheel is on the inside of the turn or is a non-steering wheel.